Perforated work pieces are required for various technical applications, in particular as cost-effective optical or mechanical filters with pore diameters in the micrometer or submicrometer range. Such applications are, inter alia, isoporous membranes, reversibly washable filters, laminators, catalyst supports, reagent supports, electrodes for batteries and fuel cells, nozzle plates, tube grids or filters for electromagnetic waves such as, for example, light or microwaves.
German Patent DE 42 02 454 C1 discloses a method for producing a perforated work piece which can be used to produce pore diameters in this region. In this method, electrochemical etching is used to form holes in and perpendicular to a first surface of a substrate wafer, made from n-doped monocrystalline silicon, so as to produce a structured layer. The electrochemical etching is performed in a fluoride-containing electrolyte in which the substrate is connected as an anode. When the holes reach a depth which corresponds essentially to the thickness of the finished work piece, the process parameters are changed such that the cross section of the holes increases, and the structured layer is detached as a platelet from which the work piece is formed.
Since the production requires that adjacent holes grow together, the shape of the perforated work piece produced corresponds to the shape of the substrate wafer. The perforated work piece is penetrated in this case continuously with pores as far as the edge. This limits the mechanical strength of the perforated work piece.
It is accordingly an object of the invention to provide a perforated work piece, and a method for producing it, which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, and has an increased mechanical strength.
With the foregoing and other objects in view there is provided, in accordance with the invention, a perforated work piece containing a substrate made from silicon and having a first region, a second region, a first main surface and a second main surface. The first region has pores formed therein that traverse the substrate from the first main surface to the second main surface. The second region has further pores formed therein which, starting from the first main surface, extend into the substrate but do not traverse the substrate.
The work piece has a substrate made from silicon in which the first region and the second region are provided. In the first region, the pores traverse the substrate from the first main surface to the second main surface. The work piece is perforated in the first region. In the second region, the pores are provided which, starting from the first main surface, extend into the substrate but do not traverse the substrate. As a result, a solid substrate material that increases the stability of the perforated work piece is present below the pores in the second region. The perforated work piece can therefore be mounted with a low risk of damage.
The thickness of the substrate in the direction of the depth of the pores is preferably greater in the second region than in the first region.
By providing a plurality of first regions, it is possible to define different filter regions, in particular for the application as a catalytic converter or a reagent support.
It is advantageous for the purpose of mounting the perforated work piece to provide the second region in an annular fashion, and to dispose the first region inside the second region. In this case, the solid edge acts in the second region as a frame for the perforated work piece.
In accordance with an added feature of the invention, in a region of the second main surface, the second region has an edge region having a surface with  less than 111 greater than  orientation.
In accordance with another feature of the invention, the pores have a first depth and the further pores have a second depth substantially equal to the first depth of the pores, and the substrate is thicker in the second region in a direction of a pore depth than in the first region.
The perforated work piece is preferably produced with the use of electrochemical etching. For this purpose, the pores whose depth is less than the thickness of the substrate are produced in the first main surface of the substrate made from silicon by electrochemical etching. The first main surface and the surface of the pores, and the second main surface, which is opposite the first main surface, are provided with a mask layer. The mask layer is structured in the region of the second main surface such that the second main surface is exposed in the first region. Using a structured mask layer as an etching mask, the substrate is subsequently etched at least as far as the bottom of the pores in the region of the exposed second main surface. The mask layer is subsequently removed, so that the pores disposed in the first region traverse the substrate from the first main surface to the second main surface.
The mask layer is preferably formed from Si3N4 or SiO2.
Etching of the substrate to form the penetrating pores in the first region is preferably performed with KOH. In the region of the second main surface, the result of this for the second region is an edge region having a surface with a  less than 111 greater than -orientation.
The electrochemical etching is preferably performed in a fluoride-containing acid electrolyte, the substrate being connected as an anode of an electrolytic cell. Since the substrate is connected as the anode, minority charge carriers move in the silicon to the first main surface, which is in contact with the electrolyte. A space charge zone is formed there. Since the field strength in the region of depressions in a surface is always greater than outside thereof, the minority charge carriers move preferentially to such depressions, which are present with a statistic distribution in every surface. This results in a structuring of the first main surface. The deeper an initially small irregularity becomes through etching, the more minority charge carriers move there because of the enlarged field strength, and the stronger the etching attack becomes at this point. The holes grow in the substrate in the crystallographic  less than 100 greater than -direction.
It is preferable to use an electrolyte with a concentration of between 2 percent by weight of HF and 10 percent by weight of HF. A voltage of between 1.5 volts and 3 volts is then applied during the electrochemical etching. This results in pores of 20 xcexcm. The diameter of the holes is preferably 2 xcexcm given a substrate doping of 5 xcexa9cm.
It is advantageous for setting the current density in the substrate to illuminate the second main surface of the substrate during the electrochemical etching.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a perforated work piece, and a method for producing it, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.